what is the use of "##" in c language?

@nerdy-me-Xm58E6 • Oct 25, 2024

Oct 25, 2024

1.5K

Though single "#" is used to concatenate two strings even in d macros..i.e.
try in dev compiler ::
#include<cstdio>
#include<conio.h>
#define delhi(s1,s2) s1#s2
using namespace std;
main()
{
char sh[]=delhi("hello","ceans");
printf("%s",sh);
system("pause");
}
but I found on net that ## is used to concatenate two strings which is wrong as # is concatenating the strings?

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nerdy_me

@nerdy-me-Xm58E6 • May 20, 2011

Though single "#" is used to concatenate two strings even in d macros..i.e.
try in dev compiler ::
#include<cstdio>
#include<conio.h>
#define delhi(s1,s2) s1#s2
using namespace std;
main()
{
char sh[]=delhi("hello","ceans");
printf("%s",sh);
system("pause");
}
but I found on net that ## is used to concatenate two strings which is wrong as # is concatenating the strings?
0
synergynext

@synergynext-FdHLd3 • May 21, 2011

the # symbol distinguishes the Pre-processor commands. for eg to include library, define a macro etc
where did u see ##? kindly share the link
0
nerdy_me

@nerdy-me-Xm58E6 • May 22, 2011

This question was asked by external in viva.I was searching its answer but couldn't find.😐😐
0
nerdy_me

@nerdy-me-Xm58E6 • May 22, 2011

I don't know about "##" .Is it signifies something in C language or it has no use?
0
vik001ind

@vik001ind-rOaCSy • May 22, 2011
There is a serious flaw in your concept. When you run your program, the output is --> hello"ceans" & not helloceans.
When we use a preprocessor directive like this one #define A a, every occurrence of 'A' is replaced with 'a' during preprocessing.
The significance of # before the parameter is that when preprocessing occurs, it is expanded into a quoted string with the parameter replaced by actual argument.
Try a simple example,
```
#include<stdio.h>
#define print(s2) printf(#s2)
void main()
{
print(hi);
}
```
It will print hi as print(hi) is preprocessed as printf("hi"). Similarly, delhi("hello","ceans") becomes "hello"""ceans"". The third & forth double quote gets nullified. Now, the combined expression is sh[]="hello"ceans"" that's why the output is hello"ceans".

In case of ##, the situation is completely different. In this case the parameters are simply concatenated.

Try this,
```
#include<stdio.h>
#define F(a,b) a##b

void main()
{
      int x =2,xy=4;
       printf("%d",F(x,y));
}
```
The output will clarify the role of ##.
0
vinsocorp

@vinsocorp-4fscJ8 • May 23, 2011

## is used to concatenate as like
ex: a=4,b=5,ab=6;
a##b compiler read as ab..
if you print the result its takes the varianle as ab;and its produces 6........
0